In the manufacture of plastic containers, it is conventional to injection mold or compression mold a container preform having a body and a finish with one or more external threads. The finish typically is molded to its final geometry, while the body of the preform is subsequently blow molded to the desired geometry of the container body. The preform may be of monolayer construction, or may be of multilayer construction in which one or more intermediate layers in the preform body may or may not extend into the finish area of the preform. U.S. Pat. Nos. 4,609,516, 4,710,118 and 4,954,376 illustrate injection molding of multilayer container preforms. It has been proposed to strengthen and rigidify the finish portion of the container by crystallizing all or selected portions of the neck finish. See, for example, U.S. Pat. Nos. 4,618,515, 4,928,835 and 6,217,818. The uncrystallized portions of the container body and finish typically are translucent, while the crystallized portion(s) of the finish contain spherulite that has a milky opaque appearance.
Molding the finish portion of the container as part of the preform presents a number of problems. For example, when the preforms are formed by injection molding, the plastic material typically is injected into a mold cavity at the closed end of the preform body, so that the material must flow along the sides of the preform cavity into the area in which the finish is molded. The finish typically requires more accurate and stable dimensioning than the body of the preform, which may limit the cycle time of the molding process. Furthermore, the finish portion of the preform is of the same material as at least the outer layers of the preform body, which limits the ability to obtain the most desirable characteristics at the finish. When the preform is of polyester construction, such as polyethylene terephthalate (PET), the finish portion of the preform can be wholly or partially crystallized to improve the operating characteristics of the finish area, particularly in hot-fill container applications. However, the requirement that the finish be of the same material as at least the outer layers of the preform body still limits the design capabilities of preform manufacture.
A finish on a hollow plastic preform or container, in accordance with a first aspect of the present invention, includes a neck that is at least partially of crystallizable polymer construction and is at least partially crystallized, and a finish ring externally secured over the neck. The neck preferably is of molded plastic construction, and preferably is stretched subsequent to molding but prior to external securement of the finish ring. In the preferred embodiments of the invention, the preform or container has an integral body, and the neck is integrally molded with the body. The finish ring preferably has at least one external thread for securement of a closure to the container after molding.
A method of making a finish on a hollow plastic preform or container, in accordance with another aspect of the present invention, includes providing a molded plastic neck that is at least partially of crystallizable polymer construction, circumferentially expanding the neck, and at least partially crystallizing the neck after expansion. A finish ring is then externally secured to the neck subsequent to expanding the neck.
The term “crystallizable polymer” in the present application refers to any polymer that is capable of being crystallized. These polymers include not only polyesters such as PET, which is the crystallizable polymer most typically used to make containers, but also polyolefins such as polyethylenes and polypropylenes, polycarbonates and polyamides such as nylons. Thus, although specific embodiments of the invention are disclosed in conjunction with polyesters—i.e., PET—the invention is by no means limited specifically to this polymer or family of polymers.
Likewise, when referring in the present application to preforms or containers that are at least partly of crystallizable polymer construction, or to preforms or containers of crystallizable polymer construction, such language refers not only to containers that are entirely of crystallizable polymer construction, but also to multilayer containers in which at least the inside and/or outside layer is of crystallizable polymer construction. Intermediate layers of the container or preform may include barrier materials against migration of gases through the container walls, layers of process regrind and/or layers that include post consumer resin. These intermediate layers may be included only in the body of the container, such that the container finish is of unlayered polymer construction, or may extend into the finish of the container. See, for example, U.S. Pat. Nos. 4,550,043, 4,609,516, 4,781,954, 4,990,301 and 5,098,274.